During the formation of neural circuits, a neuron's dendritic arbor must find its way within a molecularly complex extracellular milieu. A growing dendrite must simultaneously interact with its substrate, respond to attractive signals that guide it to its proper territory, and respond to repulsive signals from other dendrites to ensure non-redundant coverage of that territory. The molecular mechanisms regulating dendrite guidance and territory coverage, and the interactions between these mechanisms, are complex and not well understood. Repulsive cues that operate between dendrites of the same cell (sister dendrites) give rise to the phenomenon of self- avoidance. Dscam, a highly alternatively spliced immunoglobulin superfamily molecule, has been shown to regulate self-avoidance in Drosophila sensory neuron dendrites. Dscam molecular diversity appears to provide a mechanism of self- vs. non-self discrimination at the dendrite surface, such that only sister dendrites, which are likely the only branches that express the same Dscam isoforms, recognize and repel each other. The robust self-recognition and avoidance enforced by Dscam likely requires additional mechanisms that restrict dendrite arbors to the same plane of growth. An initial aim of this proposal will be to test the role and regulation of interactions between dendrites and their substrate in reinforcing the robust repulsive interactions that occur between sister branches. In other contexts, dendrites might be forced to integrate two coincident, but conflicting, extracellular signals. Such antagonism appears to characterize the relationship between dendrite self-avoidance and attractive guidance. When self-avoidance is impaired in certain sensory neurons, their dendritic arbors aggregate at specific, anatomically defined foci. One explanation for this phenomenon is that attractive guidance cues are released from these foci, and that self-avoidance normally functions to antagonize these cues. Through this interaction, self-avoidance and dendrite targeting may act together to ensure proper development of dendritic fields. As a second aim of this project, we characterize the sources and molecular nature of guidance cues acting to pattern sensory neuron dendrites. The resolution with which dendrite targeting and targeting defects can be assayed in this system will be used in the third aim to identify new genes that regulate dendrite guidance. These studies will elucidate how dendrites respond to the complex extracellular cues in their environment to ensure proper assembly of neural circuits.How dendrites acquire their proper morphologies and targets in the nervous system is poorly understood, however, aberrant dendrite morphology is associated with diverse neurological disorders, including epilepsy, mental retardation, and schizophrenia. We take genetic approaches to elucidate mechanisms that control dendrite development. Basic insights gained during this work are expected to be of significance for understanding normal and disrupted states of dendrite development and neural circuit formation.